s390/mem_detect: introduce z/VM specific diag260 call

In the case when z/VM memory is defined with "define storage config"
command, SCLP storage info is not available. Utilize diag260 "storage
configuration" call, to get information about z/VM specific guest memory
definitions with potential memory holes.

Reviewed-by: Martin Schwidefsky <schwidefsky@de.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>